This invention relates to tungsten carbides and methods of making same. More particularly, this invention relates to tungsten carbide catalysts.
High-surface-area tungsten and molybdenum carbide materials are known to possess catalytic properties similar to ruthenium, iridium, palladium and platinum. For example, high-surface-area tungsten and molybdenum carbides have been described as highly efficient catalysts for the conversion of methane to synthesis gas via steam reforming and dry reforming, and for water-gas shift reactions. Like platinum, palladium and ruthenium, tungsten carbide is also known to catalyze the oxidation of hydrogen gas at room temperature which makes it a potential catalyst for low-temperature fuel cell applications such as the PEM (polymer electrolyte membrane), sulfuric acid, and direct methanol types of fuel cells. The W2C form has been reported as being more catalytically active than the WC form in some applications.
The abundance and relatively low cost of the starting materials used to produce these carbides makes them attractive replacements for the rarer and more costly platinum metals. The main difficulty with metal carbides has been obtaining materials with sufficiently high surface areas. A high surface area is desirable for increasing the number of available catalytic sites. Original studies of preparing high-surface-area carbides used methane and hydrogen flowing over tungsten metal powder or oxides. Further improvements for tungsten and molybdenum carbides were seen in a two-step nitride-carbide formation using ammonia followed by methane. A later advancement in the art found that using ethane as a carburizing gas produced a similar effect in a one step process for molybdenum and tungsten carbides. Other attempts at producing a high specific surface included using organic intermediates. Metal carbides with surface areas as high as 200 m2/g have now been reported. Other applications for high-surface-area tungsten carbides include biomedical electrodes, e.g., electrodes for pacemakers.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to.provide a supported tungsten carbide material.
It is a further object of the invention to provide a method for forming a supported tungsten carbide material.
In accordance with one object of the invention, there is provided a supported tungsten carbide material comprising a high-surface-area support and a tungsten carbide material. The high-surface-area support is comprised of carbon or alumina. The tungsten carbide is present on the surface of the high-surface-area support and comprises tungsten and carbon. The tungsten carbide material has an x-ray diffraction pattern containing a primary x-ray diffraction peak and first and second secondary x-ray diffraction peaks; the primary x-ray diffraction peak has a reflection angle corresponding to a d-spacing of 2.39xc2x10.02 xc3x85; the first secondary x-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.496xc2x10.007 xc3x85 and a relative peak height of 25% to 40% of the peak height of the primary x-ray diffraction peak; and the second secondary x-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.268xc2x10.005 xc3x85 and a relative peak height of 35% to 55% of the peak height of the primary x-ray diffraction peak.
In accordance with another object of the invention, there is provided a method for forming a supported tungsten carbide material. The method comprises forming a mixture of a tungsten precursor and a high-surface area support and heating the mixture to a temperature from about 500xc2x0 C. to about 800xc2x0 C. in an atmosphere containing a hydrocarbon gas and, optionally, hydrogen gas for a time sufficient to convert the tungsten precursor to the tungsten carbide material.
In another aspect of the invention, the method comprises forming an aqueous solution of ammonium metatungstate, mixing the solution with a high-surface-area support, adjusting the pH of the solution to promote the formation of ammonium paratungstate, allowing the solution to set to form crystals of ammonium paratungstate, separating the solid material from the solution, and drying the solids to form a mixture of ammonium paratungstate and a high-surface area support. The mixture is then heated to a temperature from about 500xc2x0 C. to about 800xc2x0 C. in an atmosphere containing a hydrocarbon gas and, optionally, hydrogen gas for a time sufficient to convert the tungsten precursor to the tungsten carbide material.